A digital subscriber line basic rate interface (BRI) format has evolved that includes two communication transmission channels, designated B channels, and one signaling channel, designated the D channel, which may also be used to transmit customer data in a packet format. This format is commonly referred to as the 2B+D digital subscriber line format. In one example, the B channels are each 64 kbit/sec and the D channel is 16 kbit/sec. The B channels may be advantageously used to transmit text, pictures, and data, as well as speech. Consequently, the communication channels may include all logical 0's for one or more time slots. The D channel is used to transmit signaling information for the B channels.
As presently implemented, the digital subscriber line transmission distance is somewhat limited. Therefore, in order to provide service to subscribers located beyond the limited distance from an integrated services digital network (ISDN) switching office, other transmission facilities must be employed. Such facilities may include, for example, T1 carrier digital transmission lines.
As is well known, any PCM channel having all logical 0's for one or more time slots violates the logical 1's density requirement for repeaters in digital transmission lines. Additionally, all logical 0's in more than a predetermined number of consecutive channels, i.e. time slots, would cause an erroneous yellow alarm condition in T carrier systems. These conditions are undesirable and cannot be tolerated.
Prior known arrangements detect whether a channel, i.e. time slot, includes all logical 0's and insert a predetermined non-all logical 0 bit pattern in the particular channel. The fact that a substitution has been made is transmitted by causing a pre-established bipolar violation sequence. The bipolar violation sequence is detected at a receiver, and the logical 0's are reinserted in all the bit positions of the corresponding channel.
The problem with such arrangements is that the bipolar violation sequence is lost in the digital transmission hierarchy, because the bipolar digital signals are converted to unipolar form prior to multiplexing to a higher digital rate in the hierarchy. Consequently, conversion from the bipolar violation scheme to some other arrangement is required at each multiplexer in order to communicate that a channel includes all logical 0's.